In conventional borehole surveying, borehole inclination and azimuth (which, together, essentially define a vector or unit vector tangent to the borehole) are determined at a discrete number of longitudinal points along the borehole (e.g., at an approximately defined measured depth interval). Typically, no assumptions are required about the trajectory of the borehole between the discrete measurement points to determine inclination and azimuth. The discrete measurements are then assembled into a survey of the well and used to calculate a three-dimensional well path (e.g., using the minimum curvature assumption). The use of accelerometers, magnetometers, and gyroscopes are well known in such conventional borehole surveying techniques for measuring borehole inclination and/or azimuth. For example, borehole inclination is commonly derived from tri-axial accelerometer measurements of the earth's gravitational field. Borehole azimuth is commonly derived from tri-axial magnetometer measurements of the earth's magnetic field.
In making conventional borehole azimuth measurements it is assumed (i) that the actual (nominal) magnetic field of the earth is known and (ii) that the downhole tool measures only this field. Standard practice makes both assumptions. However, it is known that both assumptions are sometimes violated. Depending upon the measurement accuracy required, violation of these assumptions can be problematic. For example, the Earth's magnetic field (both the magnitude and direction of the field) is known to vary in time. Thus the actual magnetic field may not be known with sufficient accuracy. Where such variation is significant, standard practice is to use magnetic field measurements (or measurements of the variations) made at established observatories. On-site measurements of the Earth's field are sometimes also utilized; however, obtaining reliable on-site measurements can be problematic (due to the presence of magnetic interference at the rig site).
The assumption that the tool measures only the Earth's magnetic field is violated in the presence of magnetic interference. Such interference is known to cause errors in the calculated borehole azimuth values. The bottom hole assembly (BHA) itself is one common source of such magnetic interference. Motors and stabilizers (and other BHA components) used in directional drilling applications are typically permanently magnetized during magnetic particle inspection processes. BHA interference can be estimated or measured and is commonly subtracted from the magnetic field measurements. BHA interference can also be reduced through proper tool design.
Magnetic interference is also commonly encountered in close proximity to subterranean magnetic structures, such as cased well bores, or ferrous minerals in formations or ore bodies. Techniques are known in the art for using magnetic field measurements to locate subterranean magnetic structures, such as a nearby cased borehole. These techniques are sometimes used, for example, in well twinning applications in which one well (referred to as a twin well or a drilling well) is drilled in close proximity and often substantially parallel to another well (commonly referred to as a target well).
In co-pending, commonly assigned, U.S. patent application Ser. No. 11/301,762 to McElhinney, a technique is disclosed in which a predetermined magnetic pattern is deliberately imparted to a plurality of casing tubulars. These tubulars, thus magnetized, are coupled together and lowered into a target well to form a magnetized section of casing string typically including a plurality of longitudinally spaced pairs of opposing magnetic poles. Magnetic ranging measurements may then be advantageously utilized to survey and guide drilling of a twin well relative to the target well. For example, the distance between the twin and target wells may be calculated using magnetic field strength measurements made in the twin well. This well twinning technique may be used, for example, in steam assisted gravity drainage (SAGD) applications in which horizontal twinned wells are drilled to enhance recovery of heavy oil from tar sands.
While the above described method of magnetizing wellbore tubulars has been successfully utilized in well twinning applications, there is room for yet further improvement. For example, the output of the above described magnetic ranging methodology is in the form of a distance and a direction between the drilling and target wells rather than a definitive survey of the drilling well (from which a definitive well path may be derived). Moreover, in certain drilling conditions, there can be considerable noise in the magnetic ranging measurements, e.g., due to fluctuations in the measured magnetic field strength and the removal (subtracting) of the earth's magnetic field from the measured magnetic field. Such noise can result in uncertainties in the distance and direction between the twin and target wells. In SAGD operations, in which the distance and direction between the two wells must be maintained within predetermined limits, the uncertainties are at times unacceptable.
There is a need in the art for improved surveying methodologies, and in particular, methodologies that generate a three-dimensional survey of the well being drilled. There is also a need for improved magnetic surveying methods, particularly magnetic ranging methods applicable to SAGD twin well drilling operations.